Thin film transistors having a silicon-based semiconductor layer have been and being employed for switching elements of display apparatus that may be active matrix type liquid crystal display devices or organic electroluminescent devices. For instance, a gate electrode is formed on an insulating substrate and subsequently a gate insulating film is formed on the insulating substrate that includes the gate electrode. Thereafter, a semiconductor layer of amorphous silicon is formed thereon. Then, a channel protective film is formed in a center area of the upper surface of the semiconductor layer and an ohmic contact layer of n-type amorphous silicon is formed at the opposite sides of the protective film on the semiconductor channel layer. Subsequently, a source electrode and a drain electrode are arranged on the ohmic contact layer (Japanese Patent Application Laid-Open No. H11-040814).
However, since a silicon-based semiconductor material as described above is not transparent relative to visible light, conventional thin film transistors using a silicon-based material for the channel layer absorb visible light and can give rise to operation errors. Thus, conventional thin film transistors require a screening layer that prevents external visible light from coming in to make the device show a complex structure and require a complex manufacturing process.
For this reason, efforts have been and being actively paid to develop thin film transistors using a transparent conductive oxide polycrystalline thin film that contains ZnO as principal ingredient. Such a thin film shows a high mobility if compared with an amorphous silicon thin film and can be formed at low temperature. Thus, flexible and transparent thin film transistors can be formed on a substrate that may be a plastic plate or a film. Additionally, such thin film transistors do not require a screening layer because the thin film is transparent to visible light.
Now, an exemplary method of manufacturing a thin film transistor containing ZnO as principal ingredient will be described below. Firstly, a semiconductor layer of intrinsic zinc oxide is formed on a gate insulating film. Then, a protective film having a profile same as the semiconductor layer is formed thereon. Thereafter, an upper insulating film is formed on the upper surface of the protective film and a contact hole is cut through. Subsequently, either an ohmic contact layer of n-type ZnO is formed on the upper surface of the semiconductor layer exposed through the contact hole or the region thereof where the semiconductor layer is brought into contact with electrodes is subjected to a resistance-lowering treatment and then a source electrode and a drain electrode are formed on the upper surface thereof (Japanese Patent Application Laid-Open No. 2006-100760). It will be appreciated that, the above-described method requires either a resistance-lowering treatment or a step of forming an ohmic contact layer for the semiconductor layer in order to establish ohmic contact with the electrodes.
Applied Physics Letters, 89, 112123 (2006) describes a process of producing an excellently flat and uniform semiconductor layer by forming a transparent amorphous oxide semiconductor film (a-IGZO) as channel layer for a thin film transistor by magnetron sputtering at room temperature. The above-cited document describes that a thin film transistor showing a good field effect mobility of 12 cm2V−1s−1 can be obtained by doing so.
However, the above described thin film transistor device does not have an ohmic contact layer and hence there can arise a non-ohmic contact between the oxide semiconductor layer and the source electrode and the drain electrode depending on the selected electrode material.